The presence of volatile organic compounds (VOCs) in subsurface soils and groundwater is a well-documented and extensive problem in industrialized and industrializing countries. As used in this specification and its appended claims, volatile organic compounds or VOCs means any at least slightly water soluble chemical compound of carbon, with a Henry's Law Constant greater than 10.sup.-7 atm m.sup.3 /mole, which is toxic or carcinogenic, is capable of moving through the soil under the influence of gravity and serving as a source of water contamination by dissolution into water passing through the contaminated soil due to its solubility, including, but not limited to, chlorinated solvents such as trichloroethylene (TCE), vinyl chloride, tetrachloroethylene (PCE), methylene chloride, 1,2-dichloroethane, 1,1,1-trichloroethane (TCA), carbon tetrachloride, benzene, chloroform, chlorobenzenes, and other compounds such as ethylene dibromide, and methyl tertiary butyl ether.
In many cases discharge of volatile organic compounds into the soil have lead to contamination of aquifers resulting in potential public health impacts and degradation of groundwater resources for future use. Treatment and remediation of soils contaminated with volatile organic compounds have been expensive and in many cases incomplete or unsuccessful. Treatment and remediation of volatile organic compounds that are either partially or completely immiscible with water (i.e., Non Aqueous Phase Liquids or NAPLs) have been particularly difficult. This is particularly true if these compounds are not significantly naturally degraded, either chemically or biologically, in soil environments. NAPLs present in the subsurface can be toxic to humans and other organisms and can slowly release dissolved aqueous or gas phase volatile organic compounds to the groundwater resulting in long-term (i.e., decades or longer) sources of chemical contamination of the subsurface. In many cases subsurface groundwater contaminant plumes may extend hundreds to thousands of feet from the source of the chemicals resulting in extensive contamination of the subsurface. These chemicals may then be transported into drinking water sources, lakes, rivers, and even basements of homes.
The U.S. Environmental Protection Agency (USEPA) has established maximum concentration limits for various hazardous compounds. Very low and stringent drinking water limits have been placed on many halogenated organic compounds. For example, the maximum concentration limits for solvents such as trichloroethylene, tetrachloroethylene, and carbon tetrachloride have been established at 5 .mu.g/L, while the maximum concentration limits for chlorobenzenes, polychlorinated biphenyls (PCBs), and ethylene dibromide have been established by the USEPA at 100 .mu.g/L, 0.5 .mu./L, and 0.05 .mu.g/L, respectively. Meeting these cleanup criteria is difficult, time consuming, costly, and often virtually impossible using existing technologies.
One technology, which has been attempted at pilot-scale test applications, is the use of potassium permanganate (KMnO.sub.4) alone as an oxidant for in situ soil remediation. (Treatment performed in situ does not involve physical removal of the contaminated phase itself, whereas, ex situ treatment methods involve physical removal of the contaminated phase and treatment elsewhere. This has been attempted in view of KMnO.sub.4 's known capacity to oxidize target VOCs present at typical sites (e.g. trichloroethylene, dichloroethylene, and vinyl chloride). An example of such a reaction is: 2MnO.sub.4.sup.- +C.sub.2 HCl.sub.3 .fwdarw.2CO.sub.2 +2MnO.sub.2 +3Cl.sup.- +H.sup.+.
It is also well known that KMnO.sub.4 has versatile chemistry and high aqueous solubility. Once dissolved into aqueous phase, permanganate salts (such as potassium permanganate, sodium permanganate, calcium permanganate and the like) dissociate to form permanganate ions (MnO.sub.4.sup.-) that may transform to a variety of species with oxidation states of manganese in +1, +2, +3, +4, +5, +6, and +7. The most common species of manganese are manganese ions (Mn.sup.++), manganese dioxide (MnO.sub.2), and permanganate (MnO.sub.4.sup.-). The oxidation strength of (MnO.sub.4.sup.-) depends on the electron accepting capability of (MnO.sub.4.sup.-) which is pH dependent. The lower the pH, the greater the tendency of (MnO.sub.4.sup.-) to accept the electrons as indicated by the redox potential (E.sub.o) values in Eqs. 1 through 4: ##EQU1## The reactivity of KMnO.sub.4 depends on the reaction conditions and the types of organic compounds being oxidized.
While, chemically, potassium permanganate is effective at oxidizing unsaturated volatile organic compounds, currently known methods to use that ability to actually clean up a site require exceedingly large quantities of KMnO.sub.4 to overcome the natural oxidant demand exerted by the soil, thereby limiting, for a given amount of KMnO.sub.4, the percentage of KMnO.sub.4 available for oxidizing the volatile organic compounds. Large amounts of KMnO.sub.4 are thus required per unit of soil volume limiting the application of this technology due to high cost.
Another disadvantage of potassium permanganate, which has not been overcome by prior art clean-up methods, is the formation of solid manganese dioxide (MnO.sub.2) precipitates. This precipitate may result in clogging of the soil, resulting in a reduced permeability of the soil to water, reducing the hydraulic conductivity thereof, and thereby inhibiting oxidant access to the entire contaminated site rendering treatment of the soil and volatile organic compounds incomplete.
A further disadvantage of adding potassium permanganate alone and in large quantities for subsurface remediation is that it can result in the formation of soluble manganese compounds in groundwater that may exceed drinking water standards. For this and the foregoing reasons, attempts to date to use potassium permanganate for in situ applications have not been fully successful.
Early use of peroxydisulfate is reported for the purpose of organic compound synthesis. Additionally, thermally catalyzed decomposition of ammonium persulfate as a method of organic carbon digestion has been reported being accomplished at very low pH (i.e. in the vicinity of pH 2.0), but has not been thought to be useful for that purpose at higher pH. More recent publications have indicated that, under ambient temperature and uncatalyzed conditions, atrazine and PCBs may be oxidized by ammonium persulfate in aqueous solutions and in contaminant spiked soils under batch conditions. There has been no suggestion that this oxidation reaction has any application to the treatment of volatile organic compounds in contaminated soil or groundwater.
Divalent and heavy metal cation adsorption onto manganese oxide surfaces is a known phenomenon. The order of preference for selected cations to adsorb onto MnO.sub.2 surfaces is reported as follows: EQU Pb.sup.++ &gt;Cu.sup.++ &gt;Mn.sup.++ &gt;Co.sup.++ &gt;Zn.sup.++ &gt;Ni.sup.++ &gt;Ba.sup.++ &gt;Sr.sup.++ &gt;Ca.sup.++ &gt;Mg.sup.++.
Stoichiometry and rates of redox interactions with manganese dioxide and various organic compounds in aqueous solutions has been studied for some organic compounds, such as aniline and primary aromatic amines; hydroquinone; various organic acids, substituted phenols, and chlorophenols. In all of the above systems reduction of the manganese dioxide to Mn.sup.++ results in the redox couple with the organic compound being oxidized, the reaction being identified in the literature as interfacial. There has been no recognition, however, that this knowledge has application to the removal of contaminants from soil.